DE19940351B4 - Method of making a carbon sheet and an electrode - Google Patents

Abstract

A method of making a carbon sheet, comprising the steps of:
Forming a molded sheet by mixing carbon fibers and a binder;
Drying the mold layer;
Diluting a dispersion with dispersed particles of water repellent material;
Immersing the dried mold layer in the diluted dispersion;
Pulling the mold layer out of the dispersion;
Firing the mold sheet to fix water repellent material particles contained therein in the dispersion; and
Oxidize the binder to remove it from the mold layer during firing.

Description

The invention is based on a method directed to the production of a carbon layer, in particular one Carbon layer used as the starting material for an electrode of a fuel cell is used.

In general, a fuel cell comprises many cells arranged in a layer design, each cell having one Represent membrane electrode assembly (hereinafter simply referred to as "MEA") should have an ion exchange membrane, which is between two porous, electrical conductive substrates, i.e. a fuel electrode and an oxygen electrode, is arranged.

The following reaction takes place on the fuel electrode when it is supplied with a fuel gas containing hydrogen as its main component.
2H ₂ → 4H ⁺ + 4e ^-

The resulting hydrogen ions (4H ⁺ ) pass through the ion exchange membrane to reach the oxygen electrode, which is supplied with oxygen. The electrons (4e ^- ) generated in the above reaction reach the oxygen electrode by means of a connecting line. As a result, the following reaction takes place on the oxygen electrode.
4H ⁺ + 4e ^- + O ₂ → 2H ₂ O

This fuel cell works accordingly than a battery because it is used to generate electrons generated. Due to the fact that the above generation electric current on the reverse process of electrolysis of water based, except for in the liquid phase no further exhaust gases are released into the water, is such an electric power generator as cleaner or pollution-free power generator in the center of interest resigned.

To go as far as possible It is desirable to reduce air pollution there is a wide spread of electrically powered vehicles, in which fuel cells are installed. However, this desire is has not yet become a reality. The main reason for this is that the manufacturing cost of the fuel cell itself is too high. Given that the bulk of manufacturing costs of the fuel cell from the manufacturing costs of the respective electrodes it is very important to reduce electrode costs to reach.

A conventional fuel or oxygen cell is in the form of a carbon layer carrying a catalyst before, which is obtained when a water-repellent treated carbon paper carries a catalyst. Such a carbon sheet is required to have a high gas permeability, excellent electrical conductivity, high water repellency, has thermal and chemical stability and low manufacturing costs.

The generation of Water on the oxygen electrode can cause a problem if itself in the oxidant flow field collects too much water. If the oxidant stream is saturated, can be a two-phase flow occur, i.e. the oxidant stream can contain water vapor and liquid drops. liquid Water in the oxidant flow can flood the porous electrode and the oxidizer prevent the catalyst on the To reach oxygen electrode.

In order to solve such a "flooding" problem, it is known to be effective to rapidly drain the water produced by keeping the electrode moist. In the JP-A-7-130 374 (1995) and the JP-A-10-270 052 (1998) proposed solutions based on this concept.

In the first mentioned publication is for one Solid high molecular electrolysis type fuel cell Electrode in the form of a carbon layer carrying a catalyst intended. This electrode is made through the steps of manufacturing a commercially available Immerse carbon paper that has a porosity of 80% the carbon paper into a liquid in the polytetrafluoroethylene family associated Particles are dispersed, removing the carbon paper the liquid and burning the carbon paper.

The above commercially available carbon paper However, is manufactured in such a way that a carbon fiber after them with a thermosetting Material was combined under the condition of an inactive environment subjected to hot pressing at a temperature of not less than 1000 ° C, which results in high manufacturing costs. A fuel cell, in which such an electrode is used, is therefore very expensive, which means that the resulting fuel cell only difficult to convert into practical use.

In the latter publication becomes an outer surface of a Electrode substrate coated with a water-repellent material, by activating the substrate from a gas from a plasma treatment Fluorocarbon is exposed. For the use of such However, the process is a specially designed industrial one Plant or equipment required, which means that such an electrode is not is suitable for mass production and from the point of view of manufacturing costs may not be acceptable.

The main object of the invention is therefore, at a lower cost than usual a carbon layer is available to ask who regarding the water-repellent effect has a high resistance.

Another object of the invention is a method of making such a carbon sheet to use the electrode.

To solve the above tasks looks the invention a method for producing a carbon layer according to claim 1 and a method for producing an electrode according to claim 7 before.

Developments of the invention are the subclaims refer to. If the dispersant is based on water, is instead of the pulp mentioned in claim 5 as a material also a water absorption resin to absorb the dispersant such as polyacrylic acid resin, Polyvinyl alcohol resin or polyacrylamide resin can be used, and if the dispersant is oily is an oil absorption resin such as alkyl styrene or alkyl methacrylate.

• 1 eine Querschnittansicht einer Brennstoffzelle mit Elektroden, die jeweils Kohlenstofflagen gemäß einem ersten Ausführungsbeispiel der Erfindung einsetzen;
• 2 ein grafische Darstellung der Versuchsergebnisse bei dem ersten Ausführungsbeispiel und einem ersten Vergleichsbeispiel;
• 3 eine Tabelle der Tragemenge an PTFE und des elektrischen Widerstands jeder Kohlenstofflage;
• 4 eine Tabelle der Versuchsergebnisse hinsichtlich der Beständigkeit der wasserabweisenden Wirkung;
• 5 eine Tabelle der Versuchsergebnisse der Stabilität jeder Brennstoffzelle;
• 6 eine Querschnittansicht einer Brennstoffzelle mit Elektroden, die jeweils Kohlenstofflagen gemäß einem zweiten Ausführungsbeispiel der Erfindung einsetzen; und
• 7 ein eine grafische Darstellung von Versuchsergebnissen bei einem dritten Ausführungsbeispiel und dem ersten Ausführungsbeispiel.

The invention will now be described in more detail with reference to the accompanying drawings. Show it:

• 1 a cross-sectional view of a fuel cell with electrodes, each using carbon layers according to a first embodiment of the invention;
• 2 a graphical representation of the test results in the first embodiment and a first comparative example;
• 3 a table of the carrying amount of PTFE and the electrical resistance of each carbon layer;
• 4 a table of the test results with regard to the resistance of the water-repellent effect;
• 5 a table of the experimental results of the stability of each fuel cell;
• 6 a cross-sectional view of a fuel cell with electrodes, each using carbon layers according to a second embodiment of the invention; and
• 7 a graphical representation of test results in a third embodiment and the first embodiment.

First Embodiment

- Oxygen electrode training -

A slurry is made for papermaking, by carbon fiber filaments, each typically a diameter of 13 μm and a length of 3 mm, and a pulp made of wood cellulose fibers in a weight ratio of Be dispersed 1: 1 in water. The slurry is made using a process that is similar to making paper is formed into a shape layer. Then the resulting one Shape sheet cut to an electrode substrate sheet with one dimension of 160 mm × 160 mm to manufacture.

Such an electrode substrate sheet is immersed for 2 minutes in a dilute dispersion with dispersed PTFE particles, which is formed by diluting a liquid with dispersed PTFE particles with an amount of water such that the proportion of the PTFE particles 20 Weight percent. Because of the porosity of the electrode substrate layer, the diluted dispersion is thus absorbed with dispersed PTFE particles and impregnates the electrode substrate layer. The liquid with dispersed PTFE particles is manufactured by Daikin Kogyo Co. Ltd. Offered under the class name D-1 and contains 60 wt .-% PTFE particles with an average particle size of 0.20-0.40 microns and a nonionic surfactant that stabilizes the particles.

Drainage of the electrode substrate layer is achieved by placing it in an 80 ° C warm environment. After that, the resulting electrode substrate layer is left for 60 minutes at 390 ° C hot Ambient air is kept, causing the PTFE particles on the surfaces of the Carbon fibers are fixed and at the same time that in the electrode substrate layer contained pulp is removed by oxidation and gasification. In this way a carbon layer is obtained which is a water repellent material wearing. The Above mentioned PTFE is used on the electrode substrate layer for a water repellent Effect and serves instead of the removed or expelled pulp as a binder to the carbon fibers to bind to each other. The shape of the carbon layer can therefore remain unchanged.

The permeability of the carbon layer was tested according to JIS (Japanese Industrial Standard) by measuring the elapsed time for a complete escape of 300 cm ^{3 of} air in a cylinder with a bore of 10. The result was 0.8 seconds, which indicated the excellent permeability of the electrode substrate layer. The sections after pulp removal are in the form of holes that serve as gas expansion channels, thereby increasing the permeability of the electrode substrate layer. In short, the pulp serves as a hole-making agent in addition to its function as a binder.

The attempt to measure the resistance of the carbon layer was carried out in such a way that the carbon layer was sandwiched between copper electrodes, each having a diameter of 35.7 mm and an area of 10 cm ² , and these three elements under egg pressure of 196 N / cm ² (20 kgf / cm ² ) were tightened with screws. The result was 3.2 mΩ.

A paste is made by soot particles and an ethylene glycol are mixed. The paste is on the carbon layer applied by screen printing to be impregnated therein. While the Carbon layer then for 2 hours at 80 ° C in a vacuum is set, the ethylene glycol is evaporated removed so that the soot particles on remain in the carbon layer. The resulting carbon layer is again for 2 minutes in the above diluted Dispersion with dispersed PTFE particles immersed, after which a drainage The carbon layer is done by placing it in an 80 ° C warm environment is set. After that, the resulting carbon layer is left for 60 minutes at 390 ° C hot Ambient air is kept, causing the PTFE particles on the surfaces of the Carbon fibers and carbon black be fixed.

Next, a catalyst paste is prepared by using platinum-bearing carbon containing 40% by weight of platinum water, a product known as Ashiplex solution SS-1080 from Asai Kasei Co., Ltd. Offered ion exchange solution and an isopropyl alcohol with a weight ratio of 1: 1.5: 1.5: 1.5 carbon are mixed well, and applied according to a doctor knife method with a thickness of about 300 microns on one side of the carbon layer on the entire surface. The resulting carbon sheet is dried to remove the isopropyl alcohol, thereby producing an oxygen electrode. The amount of platinum carried on the oxygen electrode is about 0.4 mg per unit area (cm ² ) of the oxygen electrode.

- Fuel electrode training -

A fuel electrode is manufactured by a method similar to that for forming the oxygen electrode, except that the PTFE immersion operation is performed twice and the carbon layer of the fuel electrode uses platinum-ruthenium-bearing carbon instead of the platinum-bearing carbon. One side of the carbon layer of the fuel electrode is coated with a platinum-ruthenium-bearing carbon as a catalyst. The amount of platinum carried on the fuel electrode is about 0.3 mg per unit area (cm ² ) of the fuel electrode.

The reason why the manufacturing process the oxygen electrode differs from that of the fuel electrode, consists in the water behavior of the fuel cell. In detail must have both electrodes when starting up the fuel cell Water supplied Each electrode is therefore required to be a has water repellent effect to avoid the catalysts of the respective electrodes are under water. In stable condition however, the oxygen electrode must follow the fuel cell water production due to the chemical reaction show a water-repellent effect, while the soot near the Fuel electrode catalyst given that the hydrogen ions generated at the fuel electrode have to move with water must be hydrophilic.

Comparative Example 1

- Oxygen electrode training -

An electrode substrate layer is made by cutting a 0.23 mm thick carbon paper (offered by Toray under the designation TGP-H-060) into a 160 mm × 160 mm configuration and a carbon layer by a PTFE impregnation process which is obtained in the example 1 is similar. The permeability and electrical resistance of this carbon layer were found to be 3.7 s and 3.4 mΩ, which is worse than in the example 1 are. The test procedures and conditions are for this carbon paper and that in example 1 identical.

Similar to the procedure in Example 1 An oxygen electrode is produced by applying platinum-bearing carbon with a thickness of approximately 300 μm on the entire surface of this carbon layer. The amount of platinum carried on the oxygen electrode is about 0.4 mg per unit area (cm ² ) of the oxygen electrode.

- Fuel electrode training -

A fuel electrode is manufactured similarly to the oxygen electrode manufacturing process except that a platinum-ruthenium-bearing carbon is used instead of the platinum-bearing carbon. The amount of platinum carried on the fuel electrode is about 0.3 mg per unit area (cm ² ) of the fuel electrode.

[Evaluation method 1]

In 1 is a cross-sectional state of the structure of a single cell 20 shown, which is used for electrode testing. The cell 20 includes a membrane electrode assembly (hereinafter simply referred to as "MEA") 10 one between an oxygen electrode 1 and a fuel electrode 2 arranged ion exchange membrane 3 having. The MEA is constructed as follows: The membrane 3 is sandwiched between the electrodes 1 and 2 set that a catalyst layer 1a the oxygen electrode 1 and a catalyst layer 2a the fuel electrode 2 facing opposite sides; the resulting structure is subjected to a hot pressing process and held for 3 minutes at a temperature of 160 ° C under a holding pressure of 80 bar (80 kg / cm ² ), whereby the MEA 10 will be produced. Such an MEA 10 is between a separator 4a with an air inlet opening 5a , an air flow channel 7a , an air outlet 6a and another separator 4b with a hydrogen inlet opening 5b , a hydrogen flow channel 5b and a hydrogen outlet 6b kept causing the cell 20 will be produced.

By means of the air inlet 5a and the air flow channel 7a is on the oxygen electrode 1 air is applied under a pressure of 2.53 bar (2.5 atm) while using the hydrogen inlet 5b and the hydrogen flow channel 7b on the hydrogen electrode 2 hydrogen is applied under a pressure of 2.53 bar (2.5 atm). Steam is added to the air and hydrogen by means of a gas purging process in order to humidify them. Between the separators 4a and 4b is a variable resistor 8th intended. To evaluate the MEA according to the first exemplary embodiment and the first comparative example, the voltage or cell voltage generated and the electrical density are measured several times, the value of the resistance 8th will be changed.

[Comparison result 1]

The evaluation results for the first exemplary embodiment and the first comparative example are shown in the graph in FIG 2 shown. In this representation, the horizontal coordinate, the vertical coordinate, and the reference number 100 and the reference number 200 Current density, cell voltage, the evaluation results of the first exemplary embodiment or the evaluation results of the first comparative example. As can be seen from this illustration, the first exemplary embodiment and the first comparative example have similar characteristic curves.

In the above first embodiment can together with the pulp a dispersant belonging to the inorganic family or another inorganic binder is added to the slurry become. In terms of ply formation is a nonwoven formation process and a doctor knife method can be used, in which a carbon fiber and sludge containing binders is used.

Second Embodiment

- Oxygen electrode training -

A slurry is made for papermaking, by carbon fiber filaments, each typically a diameter of 14.5 μm and a length of 6 mm, and a pulp as a material for absorbing a Dipersants with a weight ratio of 4: 6 dispersed in water become. The mud using a process similar to that used to make paper, formed to a shape layer with a thickness of about 0.3 mm. Then the resulting mold layer is cut to an electrode substrate layer with a dimension of 160 mm × 160 mm to manufacture.

Such an electrode substrate sheet is immersed for 2 minutes in a dilute dispersion with dispersed PTFE particles, which is formed by diluting a liquid with dispersed PTFE particles with an amount of water such that the proportion of the PTFE particles 20 Weight percent. Because of the porosity of the electrode substrate layer, the diluted dispersion is thus absorbed with dispersed PTFE particles and impregnates the electrode substrate layer. The liquid with dispersed PTFE particles is manufactured by Daikin Kogyo Co. Ltd. offered under the class name D-1 and, as mentioned above, contains 60% by weight of PTFE particles.

Drainage of the electrode substrate layer is achieved by placing it in an 80 ° C warm environment. becomes. After that, the resulting electrode substrate layer is left for 60 minutes at 390 ° C hot Ambient air is kept, causing the PTFE particles on the surfaces of the Carbon fibers are fixed and at the same time that in the electrode substrate layer contained pulp is removed by oxidation and gasification. In this way a carbon layer is obtained which is a water repellent material wearing.

The above PTFE is used to on the electrode substrate layer for to provide a water-repellent effect, and serves instead of pulp removed or expelled therefrom as a binder, to bind the carbon fibers together. The shape of the carbon layer can thus remain unchanged stay.

Similar to the first embodiment, the carbon layer is finally formed into a platinum-bearing oxygen electrode. The amount of platinum carried on the oxygen electrode is about 0.4 mg per unit area (cm ² ) of the oxygen electrode.

- Fuel electrode training -

A fuel electrode is produced which is the same as that of the first exemplary embodiment Assumption is identical that the liquid with the dispersed PTFE particles in this process contains 20 wt .-% PTFE particles. The amount of platinum carried on the fuel electrode is about 0.3 mg per unit area (cm ² ) of the fuel electrode.

Comparative Example 2

It will be an oxygen electrode and a fuel electrode is manufactured so that it can, with exception that each carbon paper has a thickness of 0.23 mm (offered by Toray under the designation TGP-H-060), regarding construction and manufacturing method are identical to the second embodiment.

Comparative Example 3

It will be an oxygen electrode and a fuel electrode manufactured so that they except that each carbon paper has a thickness of 0.23 mm (offered by Nihon Carbon under the designation P-7), in terms of Structure and manufacturing method with the second embodiment are identical.

[Evaluation method 2]

 An assessment is made on the basis a measurement of the carrying amount of water-repellent material on the carbon layer, a measurement of the electrical resistance of the carbon layer and a measure of durability the water-repellent effect of the carbon layer and a measurement of stability of the MEA.

The measurement of the carrying amount of water-repellent material on the carbon layer is carried out by determining the proportion by weight of PTFE as water-repellent material with respect to the carbon fiber in the carbon layer. The electrical resistance of the carbon layer is measured such that the carbon layer is cut into a 35.7 × 35.7 mm square, the resulting carbon layer under a pressure of 392 N / cm ² (40 kgf / cm ² ) between one Is held pair of identical copper plates and an electric current is applied along the resulting structure. The measurement of the durability of the water repellent effect of the carbon sheet is carried out in such a way that the carbon sheet is floated on water at a temperature of 80 ° C., the time period until the carbon sheet begins to sink in the water.

The measurement of the durability of the MEA is carried out similarly to the above assessment 1 in such a way that the MEA is manufactured and pure oxygen under a pressure of 1.11 bar (1.1 atm) and pure hydrogen under a pressure of 1.11 bar (1.1 atm) is applied to the oxygen electrode or the fuel electrode become. The degree of utilization of oxygen and hydrogen is 80% each. Around the membrane 3 To moisten, the oxygen and hydrogen is passed through water at a temperature of 50 ° C. or through water at a temperature of 70 ° C. before such an application of the oxygen and hydrogen. The temperature of the MEA is kept at 80 ° C. Under the resulting condition there will be a voltage between the separators 4a and 4b and detects a current passing therethrough. The assessment is made on the basis of monitoring the changes in the cell voltage with the lapse of time that result when the current density is set to a constant value of 0.75 A / cm ² .

[Judgment result 2]

In the 3 Table 1 shown shows that the carrying amount of PTFE or water-repellent material in the carbon layer of the oxygen electrode (fuel electrode) according to the second embodiment is much larger than that of the second and third comparative examples. More than 160% by weight of PTFE can be carried on the carbon fiber. Despite a larger amount of PTFE or water-repellent material on the carbon layer of each electrode according to the second exemplary embodiment, the electrical resistance of the carbon layer according to the second exemplary embodiment is in each case lower than in the carbon layer according to the second and third comparative examples.

If the electrode substrate layer is in the diluted Dispersion with dispersed PTFE particles is absorbed the pulp the water that dispersed with the liquid PTFE particles are the dispersant. Along with this Water the PTFE particles reach the pulp and adhere to it on. When burning the electrode substrate layer at the temperature of 390 ° C the pulp gasified, oxidized and removed. Melt at the same time the PTFE particles, which act as a binder to bind the carbon fibers. Accordingly, the PTFE particles especially in the gaps worn, which were occupied by the pulps, and can the carbon fiber matrix structure be maintained, whereby the mutual interlocking unchanged between carbon fibers remains and there is no increase in electrical resistance. Preserves the carbon layer thus excellent electrical conductivity.

According to the in 4 shown in Table 2, which shows the durability of the water-repellent effect with each carbon layer, the carbon layers of the oxygen and fuel electrodes decrease tread in the water according to the second and third comparative examples within 5 days, whereas the carbon layers of the oxygen and fuel electrodes according to the second embodiment continue to float on the water even after 7 days have passed. From this, it can be seen that the above-mentioned difference in durability results from the difference in the carrying amount of PTFE or water-repellent material in the carbon layer of the oxygen electrode (fuel electrode) according to the second embodiment and according to the second and third comparative examples.

In the 5 Table 3 shown shows the measurement results of the cell voltage according to the second embodiment, the second comparison example and the third comparison example. Measuring the voltage between the separators 4a and 4b is carried out by taking the current density to a constant value of 0.75 for a period of 3 hours from certain points in time at which the measurement is started, 100 hours after the start and 500 hours have elapsed A / cm ^{2 is} set. The cell voltage according to the second embodiment remains essentially unchanged. The fluctuation in the measured cell voltage is very small. However, the cell voltage according to the second comparative example and the third comparative example falls in each case and fluctuates considerably during the measurement of the cell voltage. This can be due to the weaker or worse water repellency according to the second comparative example and the third comparative example.

It should be noted that the above carbon layer with excellent porous Effect also for other elements such as an electrostatic filter for air purifiers, corrosion-resistant filter and a collector for Ni-MH electrodes can be used.

Third Embodiment

In addition, hot pressing can also be used instead of the simultaneous firing and oxidizing of the electrode substrate layer. The hot pressing is carried out in such a way that the electrode substrate layer is sandwiched between a pair of male and female (not shown) for 60 minutes at an ambient temperature of 390 ° C. under a pressure of 100 bar (100 kg / cm ² ). The resulting device is in 6 shown with the oxygen electrode 1 and the fuel electrode 2 therein an oxygen flow channel 1b or a hydrogen flow channel 2 B have trained. Such a device is by means of a procedure similar to [Assessment Procedure 1] with that in

1 device shown has been compared. The results are in 7 from which it follows that the characteristic curve designated by the reference number "100" corresponds to that shown in 6 shown device with the "200" characteristic of the in 1 shown device is substantially identical.

Given that the Forming a channel in each of the separators is very expensive is that the channel is in the oxygen and fuel electrodes, respectively is trained in the manufacture of the membrane electrode assembly achieved a cost reduction.

Claims (8)

Process for the production of a carbon layer with which steps:  Forming a mold layer by mixing carbon fibers and a binder; Drying the mold layer; Dilute one Dispersion with dispersed particles of water-repellent material; immersion the dried mold layer in the diluted dispersion; Pull out the shape of the dispersion; Firing the form sheet to stick to it Particles of water-repellent material contained in the dispersion to fix; and Oxidize the binder to this during firing to remove from the mold. The method of claim 1, wherein the mold layer is a material for Absorption of the dispersant contains. The method of claim 2, wherein as an absorbent material the binder is used. The method of claim 1, 2 or 3, wherein further binders Fibers are used. A method according to claim 4, wherein the fibers are used as pulp become. The method of claim 1, wherein the burning and the simultaneous Oxidize by hot pressing the form position is achieved with a male / female pair and the resulting shape position is formed with a channel. Method of making an electrode ( 1 . 2 ) a fuel cell ( 20 ), in which a carbon layer, which was produced according to one of the preceding claims, a catalyst ( 1a . 2a ) is applied. Method according to one of claims 1 to 7, wherein the dispersant based on water.